Whipstocks are used to open a window in casing for a lateral exit to further produce an already producing zone or to reach a new zone. They are anchored to the casing at their lower end and have a long tapered surface with some curvature to guide a milling assembly laterally against the casing wall so that an elongated opening or window is made in the casing. Typically, the milling system is deployed attached to a lug at the upper end of the whipstock ramp, and that connection is severed before the mills begin to rotate. The ramp guides the milling system as the milling system moves downward along the ramp and laterally into the wall of the casing. Near the lower end of the ramp, the mill can spiral away from the ramp, as described in U.S. Pat. No. 5,474,126.
Accurate positioning of the ramp is important so that the lateral exit window is properly oriented. Sometimes, the anchor is first placed in the wellbore by using instrumentation for positioning the orienting receptacle in the anchor so that in a separate trip, the whipstock can be latched, with the correct ramp orientation, into the anchor. Some systems can run the anchor and whipstock together in a single trip and set the anchor when the desired ramp orientation is achieved. Some anchors have spiral mounting threads to engage a similar thread in the casing, so that the whipstock ramp will be properly oriented when attached to the anchor that has been advanced down the spiral mounting thread, as shown in U.S. Pat. No. 5,871,046. Similar mounting systems for anchors can be seen in U.S. Pat. No. 6,510,898 and PCT Application WO99/31348.
Horizontal and highly deviated wells pose a unique problem when using whipstocks and attempting to make a low-side exit with a downward-facing whipstock ramp. Gravity causes the upper unanchored end of the whipstock to tilt toward the low side of the horizontally placed casing. If the lateral is to be created on the high side of the casing or formation, the whipstock ramp will be upward-facing, so that if the uphole end of the whipstock is pulled by gravity toward the lower side of the lateral, subsequent re-entry into the lateral with other tools is not a problem because the tools will have ample clearance to travel onto the ramp and into the window previously made by the mills. However, if the whipstock ramp is oriented facing toward the low side of the horizontal casing and there is a need for re-entry into the lateral, the tool being advanced can either jam on the back side of the whipstock, or, if it is a mill, it can actually start milling the back of the whipstock from behind the ramp.
The present invention geometrically addresses the issue and resolves it by allowing the whipstock ramp, from upper end to lower end, to first be upward-facing and spiral to downward-facing, so that when the upper end of the whipstock is positioned flush against the bore of the casing, re-entry into the window is enabled. To position the exit in the ultimately desired low-side exit orientation, the whipstock ramp spirals around the whipstock. Those skilled in the art will better appreciate additional aspects of the invention from a review of the description of the preferred embodiment and the associated drawings, while recognizing that the full scope of the invention is to be determined by the appended claims.